1. Field of the Invention
This invention generally relates to an insulated tubular conduit and means for connecting the separate tubular conduits which has particular utility in subterranean wells.
2. Description of the Prior Art
In producing some subterranean wells, steam is injected into an injection well to increase the recovery of hydrocarbons by reducing high viscosity crude, otherwise known as "heavy crude". The lower viscosity permits the oil to flow more readily. One technique for doing this is to inject a high quantity of steam into the production zone containing "heavy crude" for an extended period of time, such as from about three to about five weeks. At that time, the viscosity of the heated crude will be reduced and it can then be pumped through a production well in communication with the production zone. The injection well may also be modified for production. A steam "flood" may also be provided by known techniques, generally through an injection well, to drive the flood and to produce hydrocarbons into a nearby production well.
One of the major problems in injecting steam into a subterranean production zone through conventional well production tubing is that the steam loses a large quantity of its heat to the well bore casing and surrounding formation as it travels to the production zone. Attempts have been made to reduce the heat loss of steam introduced in subterranean formations. One such attempt is disclosed in U.S. Pat. No. 3,511,282 issued on May 12, 1970. This patent discloses a dual wall tubular structure having insulation sealed in the annulus between the inner and outer walls by bushings respectively welded at each end between the inner wall and the outer wall. The inner wall is prestressed in tension prior to being welded to the outer wall. The space between the inner and outer walls is filled with a conventional insulating material.
Concentric walled thermal insulating conduits are also disclosed and claimed in U.S. patent application Ser. No. 264,728 filed on May 18, 1981 now abandoned and in U.S. patent application Ser. No. 272,411 filed on June 10, 1981 now U.S. Pat. No. 4,396,211. This latter patent application discloses a concentric walled tubular conduit in which the inner tubular member is flared to permit direct interconnection between the inner and outer tubular members. The inner tubular member used in the patent application 272,411 comprises a tubular member which initially has enlarged or upset ends. When the tubular member is flared with the resulting elongation of the ends, the thickness of the initially enlarged or upset sections is reduced, although the thickness still remains at least equal to the thickness of the inner tubular member intermediate to its ends. The welds between the inner and outer tubular members in concentric insulated tubing, particularly prestressed tubular conduits, are especially critical. If these welds fall the annular insulation between the inner and outer tubular conduit will be subjected to rapid deterioration, primarily due to the presence of moisture in the annulus. These wells can also be subjected to large stresses during the life of the tubular conduit string.